Process of concentrating phosphoric acid



B. W. BELLINGER PROCESS OF' CONCENTRATING PHOSPHORIC ACID Sept. 23, 1952 Filed Dec. l, 1948 3 Sheets-Sheet 1 bu nl NIS Sum

SePf- 23, 1952 B. w. BELLINGER 2,611,681

PROCESS 0F CONCIENTRATING PHOSPHORIC ACD Filed Dec. 1, 1948 s sheets-sheet 2 Y @Bgfm Sept. 23, 1952 B. w. BELLINGR PROCESS oF CONCENTRATING PHosPHoRIc ACID 5 She'ets-Sheet 3 Filed Dec. l, 1948 fr f r Patented Sept. 23, 195.2

UNITED STAT-ES PATENT OFFICE 2,611,681l 1 Y' 'l ,rnocrss or coNcENrnmNc PnosPHoRIC ACID Y Burdette W. Beilhgr, Tampa,.F1a. assigor to Tennessee Corpation, New York; N. Y., 'a corporation of New York The present invention rela-tes to' the art of concentrating phosphoric acid, and, more particularly, to an improved process of concentrating phosphoric acid efficiently on an 'industrial scale and with a minimum loss of phosphorous pentoxide. 7

It is an object of the presentinvention to pro-V vide an improved process of concent' tin phos# phoric acid from about 20% P265 to ont 50% P205 with a small loss -of- P205 which 1s evolved to the atmosphere asa dense white cloud of phosphoric acid mist.

A further object of the present invention is to provide an improved processof concentrating phosphoric acid on an industrial scale leilicie''n'i'iy and with a minimum of Pzslosses.

The invention also contemplates theprovision of an improved process of concentrating inros-` phoric acid which is capable of being carried' into practice for the production of a commercial product on an industrial scale.'-

Moreover, the present invention provides an improved process of concentrating phosphoric acid involving the exposure of the acid to be con= centrated while in 'turbulent ilowjto' th-'action of i'lame and/or hot gas whereby'concentration occurs without localized dehydration and losses.

Furthermore, the present. invention provides 1 a novel combination wetted wall evaporator' and quenching tower for carrying the Vpresent process into practice. Y

Other objects :and advantages fof their-esenti invention Will become apparentfrorn tl-'ifolow' ing description taken inlcornunctioi with thence'- companying drawings, in which: i

Fig. 1 illustrates diagrammatically an elevation view, partly in section, of equipment capable of carrying the invention into practice industrially.

Fig. 2 depicts a-vertical sectional view of the novel combination wetted wall evaporator and quenching tower;

Fig. 3 is a cross section taken on the lines 3-3 ofFig.2; and

Fig. 4 shows a top 'plan'view ofthe nove com? bination wetted Wall 'evaporatoryd' '(ivnchiig tower.

I have discovered that when phosphoric acid-is exposed to the action of ai'lairiefoi?hotgasiwith#Y out agitation, severe localized'y surface evaporation takes place with the formation of4 a partiallyfdei hydrated phosphoric acid nlm. j Upon continued exposure to the iiamefor'hot-,gas thel hyd tion `of the film goesto conplv pr,' e`sult ingf 'n' the formation f DST, and vve'AV'ft'ially",''tle'"fs'l'lkle" s claims. (c1, 21e-'1615 point.

lima'ti'on of metaphosphoric acid,'which is evolved in the gaseous state 'andLWhich becomes hydrated again. as a dense White-.cloud of H3PO4 YInistpon contact with moisture. I have. also found Ythat when the boiling point of the acid is reached; a iine spray of Heli-Oi results, which becomes `dehydrated immediately upon exposure .to the'iiame or hot gas to meta-phosphoric acid. 'The same process of dehydration takes'k place when phos-g phoric acidk comes in contact with excessively hot spots within a concentrator. system.'- in the form of a liquid, spray, lfoam, orrnist. Y

It has been discovered that .when vphosphoric acidl is exposed while in turbulentflowtogthe action of a flamev or hot gas', concentration ktaires place throughout substantially the .entire volume of acid, without localized dehydration and losses; as long as the acid remains belowy its boiling Referringv particularly-to Fig. 1',A phosphoric. acid-of approximately 22%-PzO-5 isuntroduced into the' system at inlety F at the rate of about 564 lbs, per minute where itmixes with-A the ref circulation. acidfrom the operating tank E. The re'` smting mixture or` acidis pumpediby pur'np AM from inlet Fte th novel wetted wall distributing trough R, where it l.iio'wsfcor'itinuously `and com-l pletely over the Vinside wall of the tower, asian uninterrupted acid film of variable thickness N.

Thisuninterrupted oiphosphoric `acid Valwayscovrs the inside wall soithat noibarespots occur. annular `spa'ceOf is provided forthis.

purpoe'between liotgas generator A and wetted wenevapfator B.

Heat exchange .between the acid nlm and the' het gases' within the wetted wall evaporator Waydown mewaii, acrs's'tne sie haben@ s1( of the' tower; and into an operating tai-'1k` E;

From tank E, acidv is pumped pump I spray Z-I and sprayed into -quenching tower D" which conditions the volatilized gases for sc ru bers L41, L-i2, and L-3 in the s crubberisystehinm as Well as completes the concentra-tion or the acid by drivingon` a vminor portion-Lof the total water to--b'e evaporated.- In practice-less thanl 59%,

for' example about 53 ,Y of the tota-lwater caribe evaporated in' the-said tower. ,In'the quen-ching tower, theacid'fflows acrossthe sloping bottom a portion is withdrawn as the product acid (47.6% P205). The remainder is returned to one of the operating tanks E where it is mixed with the acid from the wetted wall evaporator to produce a mixture of approximately' ll% P205. This acid mixture is continuously re-circulated throughout the system.

The feed going to distributing trough R usually runs approximately 44% P205 and is composed of acid from tank E and incoming dilute acid introduced via inlet F. It is preferred to introduce the feed tangentially at several points in order to maintain an acid velocity within the trough SiFi (gasta-2HE* (gas) :HQSiFs (liquid) This reaction occurs in all three towers 'Ir-i, L-Z,

` and 1.-3. In this manner, gases are scrubbed with capable of preventing settling out of solids or the z separation of foam. Another feature of the novel combination wetted wall evaporator and quenching tower is the prevention of the accumulation of solids within the tower by maintaining a constant now of acid down the walls and across the sloping bottoms S-l and S-2. Any separation' of solids will take place within the operating tank where they can be readily disposed of. `For this purpose-two identical operating tanks E will be provided for alternate operation and for disposal.

A direct red hot gas generator A is .arranged to re downwardlyinto wetted wall evaporator B and to liberate about 30,000,000 B. t. u./hr. It is preferred to eiect complete combustion of vthe fuel and the emergence of incandescent gases. In practice, however, complete combustion does not always occur and intermittent emergence of iiame or incandescent gases occur. However, actual tests have shown that it is not necessary to have alburning naine in the evaporator to accomplish the effective and complete evaporation and concentration described herein. Although any suitable fuel may be used, it is preferred to use Bunker C fuel oil which is the source of the heat required for evaporation. The hot gas generatoris to be equipped with two separate blowers P and Q to provide primary airA P for combustion and secondary air Q for temperature control. In this manner, an extremely flexible control of thermal conditions is provided. Temperature at the upper zone of the wetted wall evaporator can thus be adjusted tothe optimum operating conditions, which are approximately, as follows: v Y

Inlet gases to wetted wall evaporator are at approximately 2600 F., with exit gases of the order of about 35,520 C. F. M. at about 700 F.,

with a duct C velocity of about 14.2 ft./sec.linto.

quenching tower D. Exit gases from the quenching tower of the order of about 24,040 C. F. M. are at about 230 F., and a duct J velocity ofr about 14.2 ft./sec. is provided into a scrubber system comprising towers L-I, L-Z, and` L-Ss This scrubber system is designed for the recovery of fluoride gases evolved during the process of concentration of the phosphoric acid. Y

In the scrubber system using co-current and countercurrent flow, water is sprayed into tower L-3 and descends downwardly and scrubs the upwardly rising gases. From tower L-3, the water emerges at the bottom as weak iiuosilicic acid and is pumped to sprays in tower L-2. I The eiuent from tower L-2, stronger now iniluosilicic acid, combines with the eiuent from tower L-i.

This combined effluent from towers L-i and L-2 is '7 then circulated through sprays in tower L-I. The temperature of the water sprayed into tower L-3 varies between about 70 and about 80 degrees F. The approximate requirements for this Water is about one gallon of water to each 1700 acid and water sprays in the towers and the iluorides are recovered as a weak solution of HQSiFe. .L

After scrubbing, the gases are released to the atmosphere out of stack M, on the order of about 21,960' C. F. M. saturated at about 170 F. These gases are substantially free of P205 and fluorides.

Gas circulation throughout the system is accomplished by an induced draftfan K which is located between the scrubber system and exit stack M. This fan is controlled Vfor variable speed vwith a maximum capacity of about 35,000

about 187.5 tons) of about 5y0%, (e. g. aboutv 47.6%) of P205 acidper 24 hours, when carrying the` present invention into practice and when operating with the following approximate quantities of acid:

ApproximateV Item Conccntra- Approximate Rate tion f A Weak phosphoric acid feed. (22% 11205).. 564 lbs. per min. Feed to distributor trough- (44% P209-; 7,069 lbs. per min. (Max.) Discharge from Wetted (45.4% P205) 6,864 lbs. per min. (Max.)

Wall Evaporator Quencliing Tower Feed-.. (46% P209.. 2,631 lbs. per min. (Max.) Quenching D1scharge.' (47.6% P205)l l Recxrculation-; 2,273 lbs. per min. (Max.) Product 21lbs. per min.

The boiling points of Ithe various concentrations ofr phosphoric acid in impure state from the feed, trough (45%).110 the quenching Vdischarge' (47.6%) are given` `approxirriately as follows:

Concentration Boiling Point F. 44% P205 261. 5 45% P305 262.4 46% Pz0t l .263.3y 47% P205 264. 2

During the process the acid is maintained below its boiling point whereby concentrationtakes place throughoutsubstantially the entire volumev 2600 F. and is producedgby the burning of oil at the outletof ,combustion chamber A.v Additionalair is drawninto the` wetted wall tower B through the annular spa-ce by means ofan induced draft fan K (seeFig. 2). The hot gas ows down thewetted tower B where it contacts a iilm of. acidnowing down the wettedwall N. v.Approximately two-thirds of the Water evaporated vfrom the .phosphoric feed 'acid (44% P205)'. is evaporatedin this tower and the temperature ofthe combustion gases is reduced to about ,700 F. before leaving the wetted wall tower B Thy means -of the exit duct C. i

Entering the quenching tower D, the- 700 F. gas contacts a spray of phosphoric acid containing an equivalent of 46% P205 from the acid spray Z-i. The phosphoric acid is pumped to the spray Z-l by the pump I from the operating tank E. Approximately one-third of the total water evaporated from the feed acid (44% P205 equivalent) is evaporated in the quenching tower D. The hot gases containing little or no phosphoric acid mist and laden with fluoride-containing gases, such as I-I2F2 and SiF4, leave the quenching tower vD through the -duct J where they enter the iiuorine products scrubbing towers L-I, L-2, and. L-3 (seeFig. 1). The scrubber system is a combination of co-current and countercurrent type. For example, in towers L-i and L-3, the gases flow co-current with the acid spray and water spray, respectively, while in tower L-2 the gases flow countercurrent to the acid spray.

Weak phosphoric acid feed is introduced into the system through inlet valve and line F where it combines with the re-circ-ulated acids from tank E which is a combination of the acids from the towers B and D. The acids are fed to the feed distributing trough R which distributes the weak acid in an even lm over the wall N of the wetted wall tower B, in such a ratio as to produce an acid of strength equivalent to about 44% P205, by the pump M. After owing down the wetted wall the acid in tower B flows across .the inclined bottom S-I, to circulatingtank E through outlet pipe U.

From circulating tank E, the combined acids from the Vwetted wall tower B and quench tower D with a P205 equivalent of approximately 46% is pumped to the acid spray Z-l in the quenching tower D where approximately one-third of the total water is evaporated. The acid from the spray leaves the quenching tower D across the sloping bottom S-2 through an outlet pipe X where -a portion is drawn olf as Ia product through trough T-R. The remaining acid containing approximately 47.6% P205 equivalent enters circulating tank E.

The reactions occurring in the wetted Wall tower and the quenching tower may be represented as follows:

It is to be observed that in carrying the invention into practice the [acid film on the wall of the wetted wall evaporator B is continuous and is necessarily so all the time the unit is in operation. rSince large volumes of kacid are circulated over the wetted wall, the change in concentration of the acid during one pass is slight and therefore variation of vthe thickness of the aci-d film on the wetted wall due to concentration changes is slight and practically immeasurable. 'Ihe thickness of the film ycan be varied. however, by varying the output of the acid-pump supplying :acid to the wetted wall. The; thickness of this lm'is limited by the necessity of vkeeping all porticnsof, thewall wetted at all times. It is preferred to have the lm sufficientlylthick to accomplish this objective. :0n the other hand, thethicknessfmay be increased as much as desired .provided a true.` iilm .weremaintained Any breaking and raining dio-wn, of: the .acid .is not desired .and is preferably not tolerated. i 1

Although the .present invention h-as been ide- -scribed :in conjunction with "fcertain preferred embodiments, it is to be understood that modifications -and variations maybe resorted toiwit-hout `deicnairting-from the spiritl and scope ofthe invention, as those skilled inthe iart readily understand. Thus, fuel can.be-burned yat the bottom of the wetted wall evaporator and the hot burning and/or incandescent gases can be caused to flow countercurrent to the downwardly iiowing ii-lm of acid on the inside wall ofthe evaporator and then the hot gases can bevvtaken cil the top of the evaporator and carried into' the `quenching tower where' the gases can be caused 4to flow co-current with the sprayed acid. `Such variations and modifications are to be considered within the purview of the-application and the scope cf the appen-ded claims.

vI claim:

1. The improved process of concentrating 1iquid phosphoric acid from about 20% P205 to about 50% P205 with a small loss of phosphorus pentoxide which is evolved to the atmosphere as a dense white cloud of phosphoric acid mist in conventional concentration comprising introducing a feed of weak liquid phosphoric acid to be concentrated having a concentration of about 22% P205, mixing said liquid acid with recirculation Yacid having a concentration of about 46% P205 to form a liquid acid mixture having a concentration of about 44% P205, introducing said liquid acid mixturetangentially to a wetted wall distributing trough at several points in order to maintain a liquid velocity Within the trough capable of preventing settling out of solids and separation of foam, causing said liquid acid mixture to fiow from said trough uniformly over a wetted wall as an uninterrupted stream having a continuous surface iilm, subjecting said liquid acid to the direct action of hot burning gases having a high temperature of the order of about 2000 F. to evaporate a major portion of water to be evaporated and to concentrate said 1iquid acid mixture to about 46 P205 for recirculation without localized volatilization and dehydration and without substantial losses of P205 normally evolved in dense white clouds of phosphoric acid mist in conventional concentration, and spraying recirculation acid as a downwardly falling liquid spray countercurrent to rising hot gases having a temperature not higher than 700 F. to evaporate a minor portion of water to be evaporated and to produce a. concentrated liquid phosphoric acid having a concentration of about 47.6% P205, without the emission of a dense white cloud of phosphoric acid mist and without the loss of phosphorus pentoxide.

2. The improved process of concentrating liquid phosphoric acid from about 20% P205 to about 50% P205 with a small loss of phosphorus pentoxide which is evolved to the atmosphere as a dense white cloud of phosphoric acid mist in conventional concentration comprising providing a feed of a liquid'acid mixture having a concentration of aboutl 44% P205 composed of weak phosphoric acid to be concentrated having a concentrationof about 22% P205 and recirculation acid having a concentration of about 46% P205, introducing said liquid acid mixture to a wetted wall distributing trough, causing said liquid acid mixture to ow from said trough uniformly over a wetted wall as an uninterrupted stream having a continuous surfacelm, burning fuel oil to produce downwardly fired flame and hot gases and to liberate about 30 million B. t. u. per hour, adjusting primary air and secondary air in the burning of fuel oil to control the temperature of said hot gases at the inlet to a high temperature higher than about 700 F. and up to about 2600 F., subjecting said liquid acid to the direct action of said hot burning gases to evaporate a major portion of water to be evaporated, to concentrate said acid mixture to about 46% P205 for recirculation without localized volatilization and dehydration and Without substantial losses of P205 normally evolved in dense white clouds of phosphoric acid mist in conventional concentration, and to reduce the temperature of said hot gases to about 700 F. as exit gases, and spraying recirculation acid as downwardly falling liquid spray and counter-current to a rising stream of said hot exit gases having a temperature lower than 700 F. to evaporate a minor portion of Water to be evaporated, to produce liquid concentrated phosphoric acid having a concentration of about 47.6% P205 and to reduce the temperature of 'said rising stream of hot exit gases to about 230 F., without the emission of a dense white cloud of phosphoric acid mist and without the loss of phosphorus pentoxide.

3. The improved processset forth in claim 2 in which the hot exit gases are treated in a scrubber system to remove silicon tetraiiuoride and hydrogen fluoride and to produce uosilicic acid and to progressively reduce the temperature of the hot gases to about 170 F. for discharge to astaclc.

BURDETTE W. BELLINGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. THE IMPROVED PROCESS OF CONCENTRATING LIQUID PHOSPHORIC ACID FROM ABOUT 20% P2O5 TO ABOUT 50% P2O5 WITH A SMALL LOSS OF PHOSPHORUS PENTOXIDE WITH IS EVOLVED TO THE ATMOSPHERE AS A DENSE WHITE CLOUD OF PHOSPHORIC ACID MIST IN CONVENTIONAL CONCENTRATION COMPRISING INTRODUCING A FEED OF WEAK LIQUID PHOSPHORIC ACID TO BE CONCENTRATED HAVING A CONCENTRATION OF ABOUT 22% P2O5, MIXING SAID LIQUID ACID WITH RECIRCULATION ACID HAVING A CONCENTRATION OF ABOUT 46% P2O5 TO FORM A LIQUID ACID MIXTURE HAVING A CONCENTRATION OF ABOUT 44% P2O5 INTRODUCING SAID LIQUID ACID MIXTURE TANGENTIALLY TO A WETTED WALL DISTRIBUTING TROUGH AT SEVERAL POINTS IN ORDER TO MAINTAIN A LIQUID VELOCITY WITHIN THE TROUGH CAPABLE OF PREVENTING SETTLING OUT OF SOLIDS AND SEPARATION OF FOAM, CAUSING SAID LIQUID ACID MIXTURE TO FLOW FROM SAID TROUGH UNIFORMLY OVER A WETTED WALL AS AN UNINTERRUPTED STREAM HAVING A CONTINUOUS SURFACE FILM, SUBJECTING SAID LIQUID ACID TO THE DIRECT ACTION OF HOT BURNING GASES HAVING A HIGH TEMPERATURE OF THE ORDER OF ABOUT 2000* F. TO EVAPORATE A MAJOR PORTION OF WATER TO BE EVAPORATED AND TO CONCENTRATE SAID LIQUID ACID MIXUTRE TO ABOUT 46% P2O5 FOR RECIRCULATION WITHOUT LOCALIZED VOLATILIZATION AND DEHYDRATION AND WITHOUT SUBSTANTIAL LOSSES OF P2O5 NORMALLY EVOLVED IN DENSE WHITE CLOUDS OF PHOSPHORIC ACID MIST IN CONVENTIONAL CONCENTRATION, AND SPRAYING RECIRCULATION ACID AS A DOWNWARDLY FALLING LIQUID SPRAY COUNTERCURRENT TO RISING HOT GASES HAVING A TEMPERATURE NOT HIGHER THAN 700* F. TO EVAPORATE A MINOR PORTION OF WATER TO BE EVAPORATED AND TO PRODUCE A CONCENTRATED LIQUID PHOSPHORIC ACID HAVING A CONCENTRATION OF ABOUT 47.6% P2O5, WITHOUT THE EMISSION OF A DENSE WHITE CLOUD OF PHOSPHORIC ACID MIST AND WITHOUT THE LOSS OF PHOSPHORUS PENTOXIDE. 